REPRODUCTION WJ 



chromosome number in each successive generation if all cell divisions 

 occurred by mitosis. However, at some point in the succession of cell 

 divisions which constitute the life cycle of an individual, from the origi- 

 nal fertilized egg through development, growth and maturation to the 

 production of the fertilized egg in the next generation, there occurs a 

 different type of cell division, called meiosis. In the higher animals, and 

 in most of the lower ones, meiotic divisions occur during the formation 

 of gametes. Meiosis is essentially a pair of cell divisions during which 

 the chromosome number is reduced to half (Fig. 6.2). Thus the gametes 

 contain only half as many chromosomes as the somatic cells, and when 

 two gametes unite at fertilization, the normal chromosome number is 

 reconstituted. 



The reduction in chromosome number occurs in a very regular 

 way. Chromosomes occur in pairs of similar chromosomes in somatic 

 cells. As a result of meiosis, each gamete contains one and only one of 

 each kind of chromosome, i.e., one complete set of chromosomes. This 

 is accomplished by the synapsis, or longitudinal pairing, of like chromo- 

 somes and the subsequent separation of the members of the pair, one 

 going to each pole. The like chromosomes which undergo synapsis dur- 

 ing meiosis are called homologous chromosomes. They are identical in 

 size and shape, have identical chromomeres along their length and 

 contain similar hereditary factors. .\ set of one of each kind of chromo- 

 some is called the haploid number (n); a set of two of each kind is 

 called the diploid number (2n). Gametes have the haploid number (e.g., 

 23 in man) and fertilized eggs and all the cells of the body have the 

 diploid number (46 for man). A fertilized egg gets exactly half of its 

 chromosomes (and half of its genes) from its mother, and half from its 

 father. Only the last two cell divisions which result in mature, func- 

 tional eggs or sperm are meiotic; all other ones are mitotic. 



Each of the meiotic divisions has the same four stages, prophase, 

 metaphase, anaphase and telophase, found in mitosis. The chief dif- 

 ferences between mitotic and meiotic divisions are seen in the prophase 

 of the first meiotic division. Chromosomes appear as long thin threads 

 which begin to contract and get thicker. The homologous chromosomes 

 undergo synapsis, they pair longitudinally and come to lie side by side 

 along their entire length, twisting around each other. Each then be- 

 comes visibly double, as in mitosis, so that it consists of two threads. By 

 synapsis and doubling, a bundle of four homologous chromosomes, called 

 a tetrad, is formed. 



The tetrads then line up on the equatorial plate; this constitutes the 

 metaphase of the first meiotic division. The homologous chromosomes 

 now separate from one another and move to the poles. The chromosomes 

 moving to the poles during anaphase of the first meiotic division are 

 double, and at telophase each pole has received the haploid number 

 of double chromosomes. Typically, there is no interphase between first 

 and second meiotic divisions, but new spindles form (at right angles to 

 the axis of the original spindle) and the haploid number of double 

 chromosomes lines up on the equator of this spindle. Thus, the telo- 

 phase of the first meiotic division and the prophase of the second are 



